Method of manufacturing a foundry core



April 18, 1967 J. FISCHMAN ETAL METHOD OF MANUFACTURING A FOUNDRY CORE Filed May 10, 1965 SLURRY COMPRISING commw UTED REFRACTORIES,

A snucm: ACID SOL AND A GELVLING AGENT SUBJECTED TO SUB-ATMOSPHERIC PRESSURE (VACUUM) I SLURRY SUBJECTED CORE BOX TO SUB'ATMOSPHEFZIC PRESSURE SLURRI GEL-S TO GREEN CORE GREEN c022 REMOVE D GREEN CORE 'IGNTED IMPREGNATED WITH ALKANOL- WASHED WITH Al-RANOL.

CORE FIRED I l I l DRIED INVENTORS JEROME L.. FISCHMAN KENNETH H. BRINSMEAD AvronNi-r-s United States Patent ()fiice 331451 1 7 Patented Apr. 18, 1967 York Fiied May 10, 1965, Ser. No. 454,715 6 (Iiaims. (Q1. 22-194) This invention relates to foundry cores. More particularly, it is directed to improved foundry cores; and methods of manufacturing them. Even more particularly, it is directed to improved foundry cores developed from slurries comprising comminuted refractories and silicic acid sols.

It is to be noted that it is customary in making precision ceramic cores from the plastic mix, provision is made for the shrinking which occurs during drying and firing. That factor is taken care of by oversizing the core box or die. Also, it is customary practice to carry out the drying and firing for a prolonged period of time at an extremely low pace in order to avoid distortion. Consequently, several days are often required to produce a ceramic core by conventional methods.

By the method of this invention, ceramic cores can be made not only without need to provide for the shrinkage just mentioned; but also with notable reduction in the time require-d for the drying and firing. Thus, a finished core can be fabricated by the method of this invention within 6 to 8 hours as contrasted with several days which would be required to fabricate a similar core by conventional methods.

Accordingly, the principal objects of this invention are to provide foundry cores having the desired characteristics aforesaid, as well as to provide a method of manufacturing such cores.

In its essential aspects, the foundry cores of this invention are prepared from a slurry comprising comminuted refractory and a silicic acid sol wherein the green core is subjected to the treatment hereinafter described.

In the foundry industry the process of preparing ceramic bodies from comminuted refractory and a silicic acid sol (the binder) is known as the Shaw Process.

We have found that when the green cores, produced in accordance with the basic technique of the Shaw Process, are impregnated with phosphoric acid, as by immersion in the acid, after ignition, but before final firing, there is produced a core possessing markedly improved strength which is characterized by the desiderata above mentioned. F

We have further found that cores of even greater strength are obtained when the green cores, prior to treatment with phosphoric acid, are subject to high pressures. in the case of cores of simple shapes, the high pressure can be substituted by vacuum treatment.

The acid content of the phosphoric acid bath can vary from 10% to 100% of the acid, the balance of the bath, when required, being 95% ethanol (customarily denatured).

It is to be noted that an important characteristic of a core of the instant type is that it shall be readily leached out of the ultimate casting.

Cores made in accordance with the instant invention possess this important characteristic.

In general, the cores of this invention are made from suitably graded refractory material and a silicic acid sol. As is well known, silicic acid sols, constituting the binders in this art, are developed by the partial hydrolysis of alkyl silicates, for example, ethyl silicate. The instant invention utilizes especially an improved silicic acid sol (a heat hydrolyzed silicic acid sol) such as that described in the pending application of Kenneth H. Brinsmead and William B. Brown, Jr., Ser. No. 384,785, filed July 23, 1964.

In that pending application, the silicic acid sol is prepared from an alkyl silicate, e.g., ethyl silicate, which is catalytically hydrolyzed by water in the presence of a mutual solvent, the hydrolysis being carried out at an elevated temperature and including the distillation of nonaqueous volatiles with resultant formation of the silicic acid sol end product.

The gelation of the slurry can be effected by any of the agents well known in this art. Ammonia, in the form of'ammonium hydroxide solution, is quite suitable because not only is the ammonia volatile but the use thereof precludes the inclusion of alien substances in the finished core.

In manufacturing the core, the slurry, containing the gelling agent, is conveniently poured into a core box, under vibration, if desired. The slurry-filled core box is then placed into a pressure chamber before gelation of the slurry begins; and the chamber is then closed. A pressure of 40 to p.s.i., for example, 80 p.s.i., is applied for a period extending beyond the time when gelation takes place. An advantage of the pressure treatment (or vacuum treatment when used in lieu thereof in the case of cores of simple shape) is that such treatment eliminates air bubbles on critical surfaces of the core, with consequent attainment of increased strength in the finished core.

The core is left in the core box for fifteen minutes (in order to attain green strength) and then stripped, ignited and allowed to burn out. When the core cools to room temperature, it is immersed in phosphoric. acid until absorption is completed. This is manifested by the fact that bubbles cease to emerge from the core.

The phosphoric acid-impregnated core is then removed from the acid bath and placed in an alcohol bath so as to wash off the surface acid.

The alcohol-Washed core is then dried. to remove water remaining from the phosphoric acid impregnation. The time and temperature required to effect the drying are dependent upon each other and both are dependent upon the size of the core. The time involved and the temperature required to effect the drying can be readily ascertained by suitable testing.

Generally such a drying is achieved by placing the core in an oven maintained at a temperature of 400 F. to 900 F., quite suitable at about 500 F., for about one hour for each one inch of cross section.

The core is then fired at a temperature of from 1700 F. to 2000" F., preferably 2000 F.

If it be found that the fired core requires any patching or finishing, this is done by using as the patching composition, a mixture of the silicic acid sol (the binder) and finely comminuted refractory, as for example 400 mesh zircon. The patched core, desirably, is r-efired at a temperature above set forth. A core made in accordance with the invention as just described is readily leached out of the casting which encloses the same by placing the casting 'into a molten caustic soda bath. It has been found that vibration of the casting in the molten caustic soda bath is markedly helpful in effecting removal of the core.

The following are examples .in accordance with the invention:

EXAMPLE 1 A slurry is formed from the following combination of cornminuted refractories and binder:

(1) 60 parts of zircon graded as follows:

30 parts coarse sand (mesh 48 to 150); 30 parts fine sand (mesh to 250); 40 parts 400 mesh flour;

40 parts of fused silica graded as follows:

37 /2 parts 50 to +100 mesh; 22 /2 parts 200 mesh; 40 parts of dust; and

(2) A binder consisting of a heat hydrolyzed silicate (silica content, 3035%) prepared as described in Example 3 of the application of Kenneth H. Brinsmead and William B. Brown, Jr., Ser. No. 384,785.

Step 1 A slurry is formed by mixing the mixed refractories and binder in a solids/liquids proportion of 1.00 lid/100 cc. To the slurry is added a sufiicient amount of ammonium hydroxide solution (IO-% N21 to efi ect gelation within a predetermined gel time.

Step 2 The slurry is poured into a core box, desirably under vibration, and the slurry-filled core box is placed into a pressure chamber. The pressure chamber is closed and a pressure of 80 p.s.i. applied thereto for a period which extends beyond the time gelation takes place.

Step 3 (a) The core box is removed from the pressure chamber; allowed to stand for about fifteen minutes and the green core stripped therefrom on to a support; and then ignited and allowed to burn out.

(b) Alternatively, the green core when stripped from the core box is placed into denatured ethanol for not more than about fifteen minutes, removed therefrom; and then ignited and allowed to burn out.

The soak in the alcohol allows the core to attain higher green strength without loss of any of the alcohol from the binder to the atmosphere. If the soak time in the alcohol exceeds about fifteen minutes, shrinkage of the body begins to take lace due, inter alia, to the synersis of the gelled binder.

Step 4 When the core cools to room temperature, it is immersed in a solution of 50% phosphoric acid (85%) and 50% alcohol (e.g., denatured ethanol) until absorption is completed. This is manifested by cessation of bubbling. The core is then placed in an alcohol bath (e.g., denatured ethanol) where the surface acid is washed off. The alcohol-washed core is dried to remove any water contained therein. A temperature of 500 F. and a time of sixty minutes is satisfactory for a core of the following dimensions:

e.g., 12 round x 1 thick The core is then fired at 2000 F. If any patching or finishing of the fired core be required, the patching is effected With a mixture of the core hinder (the silicic acid sol) and 400 mesh zircon; and retired.

EXAMPLE 2 A slurry is formed from the following combination of comminuted refractories and binder: (1) All fused silica graded as follows:

37 /2 parts 50 to +100 mesh; 22 /2 parts 200 mesh; 40 parts of dust; and (2) A binder consisting of a heat hydrolyzed ethyl silicate (silica content -35%) prepared as described in Example 3 of the application of Kenneth H. Brinsmead and William B. Brown, Jr., Ser. No. 384,785, assigned to Avnet Shaw, Division of British Industries Corporation.

Step 1 A slurry is formed by mixing the mixed refractories in proportion of 8-10 oz./100 cc. To the slurry is added NH OH as in Example 1, to effect gelation within a predetermined gel time.

4 Step 2 The slurry is poured into a core box, desirably under vibration.

The slurry-filled core box is placed into a pressure chamber and then manipulated in accordance with the procedure as described in Step 2 of Example 1.

Step 3 The core box is removed from the pressure chamber and the core manipulated in acordance with the procedure described in Step 3(a) or 3(b) of Example 1.

Step 4 EXAMPLE 3 A slurry is formed from the following combination:

(1) All zircon graded as follows:

30 parts coarse sand (mesh 48 to 150); 30 parts fine sand (mesh 100 to 250); 40 parts 400 mesh flour.

(2) A binder consisting of a heat hydrolyzed ethyl silicate (silica content 1920%), prepared as described in Ex ample 1 of the application of Kenneth H. Brinsmead and William B. Brown, Jr., Ser. No. 301,601, assigned to Avnet Shaw, Division of British Industries Corp.

Step 1 A slurry is formed by mixing the mixed refractories and binder in a solids/liquids proportion of 2.00 lbs/100 cc. To the slurry is added a sufficient amount of NH OH, as described in Example 1, to effect gelation within a predetermined gel time.

Step 2 The slurry is poured into a core box, desirably under vibration, and the slurry-filled core box is placed into a pressure chamber. The pressure chamber is closed and a pressure of psi. applied thereto for a period which extends beyond the time gelation takes places.

Step 3 The core box is then removed from the pressure chamber and allowed to stand for fifteen minutes. The gelled core is then stripped therefrom onto a core drier and then placed in denatured alcohol, as described in Example 1.

The core is then baked out, at a temperature of about 2000" F. for about sixty minutes.

Step 4 The core, when cooled, is then painted with concentrated phosphoric acid allowing a liberal application with a paint brush (containing no nylon fibers). When this acid coating has been absorbed into the core, a second coat is applied in the same manner. The core is then fired at about 2000 F. Patching, if necessary, is carried out as described in Example 1.

EXAMPLE 4 A slurry is formed from the following combination: (1) 40 parts of Zircon graded as follows:

30 parts coarse sand (mesh 48 to 150); 30 parts fine sand (mesh to 250); 40 parts 400 mesh flour.

S 40 parts fused silica graded as follows:

37% par-ts -50 to +100 mesh; 22 /2 parts -200 mesh; 40 parts of dust. '20 parts Molochite (alumino-silicate).

-8 +40 mesh, and (2) A binder as described in Example 1 [component (2)].

A slurry is formed by mixing the mixed refractories and binder in a solids/liquids ratio of lMs-lMt lbs/100 cc.

To the slurry is added a sufficient amount of NH OH, as described in Example 1, to effect gelation within a predetermined gel time.

The vessel containing the slurry is placed in a vacuum chamber where it is subjected to a vacuum of about 28 inches/Hg for about 2-3 minutes. After the vacuum treatment, the vacuumed slurry is poured into the core box, under vibration, and undergoes gelation. The green core is then stripped from core box; ignited; and allowed to burn out. The cooled body is then manipulated as described in Step 4 of Examples 1, or 2, or 3.

EXAMPLE Step 1 A slurry is prepared as described in Example 4.

Step 2 A slurry is manipulated as described in Step 2 of Example 1.

Step 3 In this step the procedure is that described in Step 3(a) or Step 3(b) of Example 1.

Step 4 When the core cools to room temperature it is placed in a chamber and subjected to vacuum of 28 inches/Hg. A solution of 75% phosphoric acid (85%) and 25% ethanol is introduced into the chamber; and pressure of 40-80 p.s.i. applied thereto for about 5-10 minutes.

The core is removed therefrom; placed in an alcohol bath to wash out the acid, then dried at 500 F. for about 60 minutes; and fired at 2000" F.

EXAMPLE 6 A core is prepared by the procedure described in Example 4 with the following modification: High melting glass supporting rods are introduced into the slurry-filled core box so as to become embedded therein upon gelation. It is desirable that care be taken to prevent the glass rods from coming into contact with the sides of the core box.

Example 1 of the application of Brinsmead and Brown, Ser. No. 384,785 mentioned above, describes the preparation of the binder as follows:

A three-necked flask, fitted with a thermometer, a reflux condenser, and a dropping funnel is charged with 1157 parts of ethyl silicate 40 and 774 parts of 90% denatured ethanol.

The mixture is maintained under agitation by magnetic stirring and heated until the temperature reaches 80 C. The heating is discontinued; and hydrolysis initiated by slowly dripping in through the separatory funnel 90 grams of hydrochloric acid (0.221 N). The drip speed suitably should be from 2.2 ml. to 5.5 ml. per minute. If the drip speed exceeds 5.5 ml. per minute, it becomes difiicult to maintain the reaction mixture under reflux.

After the addition of the acid and the attainment of an exothermic peak, which indicates that the reaction has been completed, the reflux condenser is replaced by a fractionating column and a suitable condenser. The reaction mixture is then heated and the desired amount of alcohol is distilled off; in this instance, 405 parts of ethanol. This produces a binder having a silica content of 31% by weight.

It has been found that the removal of the alcohol should be carried out within 1-6 hours inasmuch as 1-6 hours appears to be the optimum heating time at refluxing temperature after completion of the hydrolysis.

If the volume of alcohol to be distilled off is greater than that which can conveniently be removed in 1-6 5 hours, then the distillation can be carried out under reduced pressure at a lower temperature.

Example 3 of the application of Brinsmead and Brown, above mentioned, describes the preparation of the binder as follows:

The reaction vessel, fitted with thermometer, a reflux condenser to which is added a take-off apparatus, and a dropping funnel, is charged with 300 lbs. of ethyl silicate 40, 178.5 lbs. of 95% ethanol. The dropping funnel is filled with 22 lbs. of water. The mixture in the reaction vessel is heated until it begins to boil; the heat is turned off, and then 0.27 lb. of concentrated sulfuric acid, for example, 98%, is added very slowly, the addition being carried out at a rate of no more than 0.05 lb. per minute. When the reaction subsides, the valve in the take-off assembly is opened and 156.5 lbs. of ethanol is removed.

There is thus obtained 343.5 lbs. of a binder having a silica content of 37.7%, by weight,a viscosity of from 8 to 9.5 centipoises. The binder possesses a minimum shelf life, at 104 F., of 22 days.

It will be understood that the foregoing description of the invention and the examples set forth are merely illustrative of the principles thereof. Accordingly, the appended claims are to be construed as defining the invention with the full spirit and scope thereof.

We claim:

1. Method of manufacturing a foundry core which comprises:

( 1) forming a slurry comprising comminuted refractories, a silicic acid sol and a gelling agent;

(2) pouring the slurry into a core box; and allowing it to gel;

(3) removing the green core from the core box;

(4) igniting the green core and allowing it to burn out;

(5) impregnating the burned core with phosphoric acid; and

(6) firing the core.

2. Method of manufacturing a foundry core which comprises:

( 1) forming a slurry comprising comminuted refractories, a silicic acid sol and a gelling agent;

(2) pouring the slurry into a core box;

(3) subjecting the slurry to a pressure different from atmospheric to effect elimination of surface bubbles; and allowing it to gel;

(4) removing the green core from the core box;

(5) impregnating the green core with a lower alkanol;

(6) igniting the alkanol-impregnated green core and allowing it to burn out;

(7) impregnating the burned core with phosphoric acid;

(8) washing the phosphoric acid-impregnated core With a lower alkanol;

(9) drying the alkanol-washed core; and

(10) firing the core.

3. Method of manufacturing a foundry core which comprises:

(1) forming a slurry comprising comminuted refractories, a silicic acid sol and a gelling agent;

(2) pouring the slurry into a core box;

(3) subjecting the slurry to elevated pressure; and

allowing it to gel while under such pressure;

(4) removing the green core from the core box;

(5) impregnating the green core with a lower alkanol;

(6) igniting the alkanol-impregnated green core and allowing it to burn out;

(7) impregnating the burned core with phosphoric acid;

(8) washing the phosphoric acid-impregnated core with a lower alkanol;

(9) drying the alkanol-washed core; and

(10) firing the core.

4. Method of manufacturing a foundry core which comprises:

(1) forming a slurry comprising comrninuted refractories, a silicic acid sol and a gelling agent;

(2) subjecting the slurry to subatmospheric pressure;

(3) pouring the slurry into a core box, and allowing it to gel;

(4) removing the green core from the core box;

(5) impregnating the green core with a lower alkanol;

(6) igniting the alkanol-impregnated green core and allowing it to burn out;

(7) impregnating the burned core with phosphoric acid;

(8) washing the phosphoric acid-impregnated core with a lower alkanol;

(9) drying the alkanol-washed core; and

(10) firing the core.

5. Method of manufacturing a foundry core which comprises:

(1) forming a slurry comprising comminuted refrac tories, a silicic acid sol and a gelling agent;

(2) pouring the slurry into a core box; and allowing it to gel;

fa) (3) removing the green core from the core box; (4) igniting the green core and allowing it to burn out; (5) impregnating the burned core with phosphoric acid; (6) washing the phosphoric acid-impregnated core with 5 a lower alltanol;

(7) drying the alkanol-washed core; and (8) firing the core. 6. In the manufacture of a foundry core as defined in claim 1, the further step of firing the phosphoric acid- 10 impregnated core at a temperature of about l700 to References Cited by the Examiner UNITED STATES PATENTS 2,359,321 10/1944 Lindemuth 106-38.23 2,390,765 12/1945 York Et al. 106-3823 2,563,643 7/1951 De Ranek 22-194 2,806,270 9/1957 Shaul 22-196 2,875,073 2/1959 Gogek 22 194 3,160,931 12/1964 Leach 22 194 3,179,990 4/1965 Freeman 22-194 3,209,421 10/1965 Shephard 10638.3 X 3,213,497 10/1965 Scott 22 192 J. SPENCER OVERHOLSER, Primary Examiner.

E. MAR, Assistant Examiner. 

1. METHOD OF MANUFACTURING A FOUNDRY CORE WHICH COMPRISES: (1) FORMING A SLURRY COMPRISING COMMINUTED REFRACTORIES, A SILICIC ACID SOL AND A GELLING AGENT; (2) POURING THE SLURRY INTO A CORE BOX; AND ALLOWING IT TO GEL; (3) REMOVING THE GREEN CORE FROM THE CORE BOX; (4) IGNITING THE GREEN CORE AND ALLOWING IT TO BURN OUT; 